With the increasing demand on the performance of capacity and flexibility of the optical communications system, the coherent optical communication technology has become more and more important. In comparison with incoherent technology (such as on-off key, OOK) or auto-coherent technology (such as differential quadrature phase shift keying, DQPSK), the coherent technology has the following advantages: 3 dB of optical signal-to-noise ratio (OSNR) gain; capability to use more efficient modulation technologies (such as quadrature modulation QAM) to enhance transmission capacity; and convenient use of electric equalization technology to respond to channel variations, and to lower the production cost, etc. Like the case in electric coherent technology, it is also necessary for an optical coherent receiver to recover carrier phase. Currently, carrier phase recovery in the optical coherent receiver is generally achieved via digital technology. For instance, in “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation”, (Journal of Lightwave Technology, Vol. 24, No. 1, January 2006, pp 12-21), D. Ly-Gagon et al. propose a method based on Mth power, and in “Multiplier-free Phase Recovery for Optical Coherent Receivers” (OWT4, OFC2008), Z. Tao et al. propose a method based on data predetermination, etc. Due to the presence of optical noises in channels and electric noises in the receiver, it is necessary for the digital phase recovery technology to filter the received signal to remove the influence of the noises so as to obtain the precise carrier phase. Thus, performance of phase recovery is closely related to parameter selection of the filter. As well known, optimization of parameters of the filter should be directed to the noises and the filtered signal (optical carrier phase in this context). However, variation of the optical carrier phase is decided by many factors (such as laser characteristics in the transmitter/receiver, configuration of fiber channels and signal powers of adjacent channels) in an actual transmission system. These factors are not invariant and difficult to be obtained by real-time detection.
FIG. 1 shows a prior art optical coherent receiver that makes use of a digital phase recovering apparatus. In FIG. 1, the optical 90° frequency mixer 102, photoelectric converters (O/E) 104 and 105, analog-to-digital converters (ADC) 106 and 107, and laser 103 compose the front end 111 of the optical coherent receiver. The function of this section is to convert the received optical input signal 101 into a baseband electric signal 108. The baseband electric signal 108 can be expressed as I+jQ=exp(jφd+jφ0)+n, where n indicates noise. In general circumstances, the baseband electric signal 108 not only contains data information φd, but also contains phase offset φ0 between carrier and local oscillation (laser 103). Function of the digital phase recovering apparatus 109 is to remove the phase offset φ0 in the baseband electric signal 108. Output from the digital phase recovering apparatus 109 is the data information φd, and the data recovery device 110 recovers the transmitted data in accordance with the input data information. As can be seen from the above, the digital phase recovering apparatus 109 is a very important part in the optical coherent receiver.
The prior art digital phase recovering apparatus can be represented by the structure as shown in FIG. 2. The data removal modulator 204 performs Mth power calculation on the input digital baseband electric signal 201 (108) to remove data phase modulation (where M is the order of the digital phase modulation, for example, M=4 with respect to QPSK), so as to obtain a demodulated signal 206, namely exp(jMφ0)+n*, where n* indicates the noise after Mth power calculation. In general circumstances, variation of the phase offset is slower than variation of the noise. Consequently, it is possible to use an averager 205 to remove the influence of the noise. The argument calculator 210 obtains a value 211 of the phase offset in the range [−π/M, π/M] on base of the output 207 of the averager 205, the unwrapper 212 unwraps the value 211 as a value in the range [−π, π], this value is the estimated value {circumflex over (φ)}0 of the phase offset, and the subtracter 208 finally subtracts the estimated value {circumflex over (φ)}0 of the phase offset from the symbol phase obtained by the argument calculator 202 to thereby obtain the signal 209 whose phase has been recovered.
As can be seen from the above, working performance of the phase recovering apparatus is mainly dependent upon the design of the averager. The averager 205 can perform arithmetic averaging in segments, and can also perform sliding averaging. There are currently some methods that adaptively adjust the length as used by the averaging to optimize the performance of phase recovery, for instance, “Adaptive optimization for digital carrier phase estimation in optical coherent receivers”, pages 121-122, 2008 Digest of the IEEE/LEOS Summer Topical Meetings.
However, in the process of researching on the present invention the inventor of the present application has found that such optimization is directed only to the adjustment of the length used in the averaging, while it does not involve the relative relationship of phase offset on each symbol within the length. Accordingly, the optimal performance cannot be obtained by such optimization.
A technique is therefore currently needed to adaptively optimize the filter coefficient in digital phase recovery, so that the digital phase recovering apparatus of the optical coherent receiver operates at the best status.